Field of the Invention
The present invention relates to an electron microscope and method of adjusting it.
Description of Related Art
A monochromator is generally composed of an energy filter for dispersing an electron beam and energy-selecting slits. An electron beam impinging on the monochromator is spectrally dispersed by the energy filter and becomes a spectrum corresponding to an energy distribution of the beam on the surface of an energy-selecting slit located on an energy dispersive plane. By using this energy-selecting slit for the spectrum, only a certain energy width of the electron beam corresponding to the slit width of the energy-selecting slit passes through the slit. As a result, the beam is monochromatized.
Since a monochromator is so designed that a part of an electron beam is selected by an energy-selecting slit, it is inevitable that the amount of electrical current of the beam impinging on a sample will decrease. This, in turn, will lead to a decrease in the brightness of the electron beam. As a result, the performance of the electron microscope will be affected greatly. Therefore, it is necessary that the decrease in the amount of electrical current of the beam due to the monochromator be suppressed to a minimum. In order to secure high energy resolution being one type of fundamental performance of the monochromator and to suppress decreases in the brightness of the electron beam, it is necessary to optimize the position of the energy-selecting slit relative to the spectrum of the electron beam such that the amount of electrical current of the beam passing through the energy-selecting slit is maximized.
Conventional positional adjustment of an energy-selecting slit in a monochromator has depended much on the human operator. The operator judges, depending on direct observation, whether the amount of electrical current of the electron beam that has passed through the slit has increased or decreased from the degree of brightness of the beam, and moves the position of the slit to maximize the amount of current of the beam passing through the slit.
On the other hand, one conceivable method of adjusting the position of the energy-selecting slit in a monochromator consists of measuring the beam passing through the slit by a Faraday cup and an ammeter and adjusting the position of the slit. In this case, the beam passing through the slit is fully absorbed by the Faraday cup and thus it is impossible to directly observe the shape of the beam. An adjustment of the monochromator needs both making an adjustment of the energy filter while directly observing the shape of the beam and an adjustment of the position of the energy-selecting slit. Whenever an operational condition of the energy filter is varied, the Faraday cup is inserted and the amount of electrical current of the beam is measured. This operation is cumbersome to perform and imposes further operational burden on the operator. Furthermore, it takes long to adjust the monochromator. Consequently, it cannot be said that the instrument is easy to use.
Accordingly, JP-A-2011-129257, for example, discloses a method of controlling the position of an energy-selecting slit in a monochromator in such a way that the monochromator can be adjusted in a short time by detecting the electrical current flowing through the slit and controlling the position of the slit such that the detected amount of electrical current is minimized.
However, in the technique of JP-A-2011-129257, the position of the energy-selecting slit is adjusted by mechanically moving it. Therefore, when the energy-selecting slit has just started to move, rattling in a reverse direction moves the slit in a direction reverse to the intended direction. In this way, it is difficult to smoothly move the energy-selecting slit. Accordingly, with the technique of JP-A-2011-129257, it may take long to adjust the position of the energy-selecting slit.